The amount of energy transmitted by an electronic component into a board on which the component is mounted depends on a number of factors including, for example, the materials used in the component's package, construction of the package, materials used in the board, and construction of the board. Similarly, the energy transmitted by the component into air adjacent to the component is also a function of a number of variables, such as the air's velocity and the construction of the component's package. Calculation of the amount of energy transmitted by a component into the air and into the board is extremely complex and time consuming.
However, such calculation is helpful to determine the performance of the component in a system. One parameter useful in calculating the destination of the energy emitted from a package is the thermal resistivity of the package. The thermal resistivity of an electronic component's package is typically measured with the package mounted on a board, the board having either ground and power planes, or only conductive traces on the top and bottom side of the board. The thermal resistivity can also be measured with the package suspended inside a box.